Vehicle load responsive valve devices are employed for controlling the degree of fluid pressure supplied to vehicle brake cylinders in accordance with the load condition of the vehicle. Such valve devices usually operate responsively to the amount of vehicle spring deflection, either mechanical or air springs, for supplying brake-applying pressure to the brake cylinders at a degree commensurate with vehicle load.
In the case of a motor vehicle having dual rear axles, one of which is a driving axle and the other of which is a trailing axle, certain problems relating to the respective braking forces for each axle are presented due to dynamic axle load displacement.
On a vehicle having a front and a single rear axle, dynamic axle load displacement also occurs during vehicle deceleration or braking. Due to dynamic axle load displacement, normally from the rear axle to the front axle, which is proportional to the rate of vehicle deceleration, a greater braking force is normally applied to the front axle relative to the rear axle. As the vehicle continues to decelerate, however, the degree of axle load displacement lessens, and, therefore, the degree of braking pressure supplied to the rear axle should be increased, but at a rate commensurate with the rate of deceleration. For this purpose a braking pressure regulating valve device is provided and operates responsively to vehicle rate deceleration for providing a relatively low increase of braking pressure to the rear axle within a range of lower rates of vehicle deceleration and a sharply higher increase of braking pressure to the rear axle during higher rates of vehicle deceleration.
With a vehicle having dual rear axles, however, not only must the factor of dynamic load displacement among the several axles be considered, but the additional matter relating to the load conditions of the vehicle, that is, whether empty or loaded, also becomes a factor to be considered. Variation in stress on the front axle of the vehicle, whether loaded or not, is substantially less than that on the rear dual axle assembly, and, therefore, this invention is primarily concerned with the problems related to the dual axles.
Since the braking apparatus of a vehicle is normally set up to provide adequate braking for a loaded vehicle, such braking force (that is, that for a loaded vehicle) in the case of an empty vehicle and taking into account dynamic axle load displacement, may be excessive for the trailing axle wheels and cause them to lock. As a result, with the trailing axle wheels in a locked state, which in itself is a dangerous condition, the driving axle wheels must assume substantially the entire braking function, thereby resulting in more rapid wear of the brake linings thereon. One solution for this problem is to provide a load responsive brake pressure regulating valve for each axle, which would be an obviously expensive solution.